1. Field of the Invention
The present invention relates to a push-button signal receiving device for receiving a push-button signal and identifying a dialed number based on the received signal, and a gain control method for a variable gain amplifying circuit provided in such a push-button signal receiving device.
2. Description of the Related Art
In analog telephone communications, push-button signal (hereinafter abbreviated as PB signal) is used as a selection signal whereby a telephone terminal specifies a target of connection. In such communications using the PB signal, one low-frequency signal is combined with one high-frequency signal to create one of 16 different signals, based on which a dialed number is identified. The combination of low- and high-frequency signals is judged to be a valid selection signal if continuously sent out for a fixed time or longer in response to a single dialing operation. Also, between dialing operations, a time period (pause) needs to be provided which is equal to or longer than a prespecified time and in which no signal is present.
In such analog telephone communications, transmit power is prescribed with which the PB signal is transmitted from a telephone terminal. However, the signal level varies due to transmission loss or the like on the line leading to a receiving device that receives the PB signal, making it impossible for the receiving device to determine the minimum input level. Accordingly, to secure a wide dynamic range for the receive signal, there has conventionally been used a PB signal receiving device equipped with an auto gain control (AGC) circuit including an amplifying section and a level detecting section.
FIG. 5 illustrates a schematic configuration of a conventional PB signal receiving device.
The conventional PB signal receiving device 30 shown in FIG. 5 comprises an AGC circuit 31 including a variable gain amplifier 31a for amplifying the input signal and a level detector circuit 31b for detecting the signal level, filter circuits 32 and 33 for separating the input signal into low- and high-frequency signals, and a dialed number identification circuit 34 for detecting the frequencies of the low- and high-frequency signals to identify the dialed number.
In the AGC circuit 31, the level detector circuit 31b detects the level of the output signal of the variable gain amplifier 31a and notifies the amplifier 31a of the detected signal level. In accordance with the value of the signal level detected by the level detector circuit 31b, the variable gain amplifier 31a amplifies the input signal to control the signal level so that the level of the PB signal input to the dialed number identification circuit 34 may fall within a prescribed operating range.
The filter circuits 32 and 33 remove signals of high- and low-frequency bands, respectively, from the output signal of the AGC circuit 31. The dialed number identification circuit 34 detects the frequencies of the output signals from the filter circuits 32 and 33 and, on detecting valid low and high frequencies, identifies and outputs a dialed number specified by the detected low and high frequencies.
The process carried out by the dialed number identification circuit 34 will be described in more detail. In response to the timings of extraction of both the low- and high-frequency signals for frequency detection, the dialed number identification circuit 34 measures the continuance time and discontinuance time of the extracted signals, to determine whether or not the received PB signal is a valid selection signal. FIG. 6 illustrates the received PB signal and validity determination therefor.
As shown in FIG. 6, the dialed number identification circuit 34 detects the frequency of the PB signal input to the PB signal receiving device 30. On detecting both the low- and high-frequency signals, the circuit 34 starts to measure the continuance time of the detected signals, and when a continuance criterion time Ton has elapsed, the circuit 34 judges that the PB signal is valid and outputs a completion signal indicating completion of encoding of the PB signal. Also, if the signals discontinue and thus are not detected, the discontinuance time is measured, and when a discontinuance criterion time Toff has elapsed, the PB signal is judged invalid and the output of the completion signal is stopped. When a signal with the same frequencies is received thereafter, it is judged that the received signal is a different selection signal. The dialed number identified based on the detected frequencies is output at the output timing of the completion signal only for a period valid as the selection signal.
In FIG. 6, the measurement of the continuance time and discontinuance time of the signal is started at the reception start and stop timings, respectively, of the PB signal. In practice, however, the timing for starting the time measurement is delayed in some degree because of the time needed for the frequency detection, etc.
In the PB signal receiving device 30 described above, the input PB signal level is adjusted by the AGC circuit 31 so that the signal level input to the dialed number identification circuit 34 may fall within an operating range of the frequency detection process, irrespective of variations in the received PB signal level attributable, for example, to transmission loss caused in the course of transmission from the telephone terminal, thereby permitting accurate frequency detection.
Also, in the aforementioned configuration of the PB signal receiving device 30, the signal level is detected from a composite wave of low and high frequencies to control the signal level of the input signal, and as a consequence, distortion occurs in the control current for the variable gain amplifier 31a, possibly causing error in the frequency detection. The problem is solved by a PB signal receiving device disclosed in Unexamined Japanese Patent Publication (KOKAI) No. 54-95108. According to the publication, signal level is detected based on the output signals of filter circuits for separating the input PB signal into low- and high-frequency signals, and in accordance with the detected value, the amplification gain of an input-stage variable gain amplifier is controlled to thereby solve the problem.
Meanwhile, in the AGC circuit 31 of the above PB signal receiving device 30, the gain of the variable gain amplifier 31a does not vary exactly following changes in the PB signal level detected by the level detector circuit 31b but a certain length of time is required for the follow-up. Consequently, error can occur in the measurement of time of the extracted signal by the dialed number identification circuit 34, possibly causing a situation where validity of the PB signal fails to be accurately determined.
FIG. 7 is a timing chart showing signals appearing in various parts of the PB signal receiving device 30.
Part (A) in FIG. 7 shows an input signal S31 input to the AGC circuit 31, wherein reception of the input signal S31 by the AGC circuit 31 is temporarily interrupted from timing T701 to timing T702 due to discontinuance of the signal for some reason or due to inclusion of noise in the signal, for example. The discontinuance time of the input signal S31 from T701 to T702 is shorter than the discontinuance criterion time Toff.
Part (B) in FIG. 7 shows a gain A32 of the variable gain amplifier 31a, wherein the gain A32 does not exactly follow the input start and stop timings of the input signal S31 and is varied stepwise.
Part (C) in FIG. 7 shows a signal derived by amplifying the input signal S31 by the variable gain amplifier 31a and S33 output from the AGC circuit 31. In (C) of FIG. 7, the level Gmax represents a maximum value of the operating range in which the frequency can be detected when the signal S33 is input to the dialed number identification circuit 34 through the filter circuits 32 and 33.
Part (D) in FIG. 7 shows the completion signal output from the dialed number identification circuit 34.
In FIG. 7, before timing T701, the completion signal is at H level because of the input of the input signal S31, and the gain A32 of the variable gain amplifier 31a is at a minimum value. The input signal S31 discontinues at timing T701, whereupon the level detector circuit 31b of the AGC circuit 31 detects the discontinuance of the signal, and the gain A32 of the variable gain amplifier 31a is gradually increased. At this time, since the level of the input signal S31 is “0”, the signal S33 remains at “0” and the dialed number identification circuit 34, wherein the signal input has discontinued, starts to measure the discontinuance time from timing T701.
Subsequently, the signal S31 is again input at timing T702. Thus, the level detector circuit 31b of the AGC circuit 31 detects the signal input and the gain A32 of the variable gain amplifier 31a is gradually decreased down to the aforementioned minimum value. Consequently, while the gain A32 is decreased to the minimum value, the input signal S31 is amplified as indicated by the signal S33.
The dialed number identification circuit 34 is unable to perform the frequency detection process and thus to detect the input of the input signal S31 until timing T704 at which the level of the amplified signal S33 drops below the level Gmax. If the measured discontinuance time reaches the discontinuance criterion time Toff at timing T703 between T702, at which the signal S31 is actually input again, and T704, the dialed number identification circuit 34 judges that the PB signal is invalid, and stops outputting the completion signal.
Then, the dialed number identification circuit 34 again extracts the PB signal at timing T704 at which the level of the signal S33 drops below the level Gmax, and starts to measure the continuance time. At timing T705 after a lapse of the continuance criterion time Ton, the circuit 34 judges that the PB signal is valid, and again outputs the completion signal.
In the above exemplary operation shown in FIG. 7, even though an actual pause time T from T701 to T702 is shorter than the discontinuance criterion time Toff, the dialed number identification circuit 34 recognizes a time Tw from T701 to T704 as a pause time and stops outputting the completion signal. Thus, where the gain A32 of the variable gain amplifier 31 varies gently, error occurs in the start timing of pause detection by the dialed number identification circuit 34 and the signal can be judged invalid because of a merely instantaneous discontinuance, giving rise to a problem that the dialed number is erroneously recognized.